The cast of "Castle" recently celebrated filming their 100th episode, which airs on Monday, April 1, and OTRC.com got a little inside scoop.

Since the show debuted in 2009 the relationship between New York City homicide detective Kate Beckett (Stana Katic) and novelist Richard Castle (Nathan Fillion) has grown, along with the cast of characters on the show.

OTRC.com had a chance to catch up with the cast members of the show, and series creator Andrew W. Marlowe, to talk about the big milestone. We also learned a few things about the episode and about the cast as well.

So check out four things we learned from our "Castle" 100th episode set visit below. (Warning: Spoilers ahead!)

1. The 100th episode has a Hitchcock vibe.

Fillion shed some light on the plot for the 100th episode and explained, "Castle broke his leg and out of sheer boredom, he's participating in a little voyeurism. He's watching the people in this building across the street, and our episode becomes a bit of a 'Rear Window' episode."

"I witness a murder and no one seems to think that I saw what indeed I actually saw," Fillion added. "Everyone's got a reason why ... a reason not to believe me. So I take matters into my own hands."

Katic also told us that Beckett has to balance her time a little in the episode.

"Castle's broken his leg and he sees a murder outside of his loft window and Beckett has to jump between babysitting Castle and taking care of a real murder out there in New York City," she said. "There's a car chase and an explosion and there's a love story and it's unrequited. We have a scandal. You should watch."

2. Marlowe said 125 actresses tried out for the role of Detective Beckett but it was Katic who had that special something with Fillion.

"I think the show all starts with the chemistry between Nathan and Stana and we were really blessed in order to get Nathan. Nathan was great," Marlowe said. "We read with 125 different actresses and we were looking for that ineffable something, that chemistry, and when Stana came in, we found it."

"Without that we wouldn't have a show," he added. "So that's what ... if I could point to one thing, it's that. That's what people tune in to see every week."

3. What's next for Castle and Beckett's relationship? The couple has a few questions to answer.

Marlowe told OTRC.com about the couple's relationship, "I think the next step for this couple is figuring out what are they going to do next."

"They're coming up on a year this spring, what are we doing next? What is this relationship? Is this real? Are we taking the next step? How invested are you in me? How invested am I in you? So I think we'll be wrestling with some of those big questions," he added.

"To watch Molly grow up and to just get to know everybody and let this become my secondary family and we're all still here doing what we love, it's a true blessing," she said. "You go to sets and you get along with a few people and with other people it's like, 'Oh hey good morning,' and you go on about your business."

"But here we all love and kiss and hug on each other and do special things like get the ice cream man out here with the truck or get the coffee man out here with the truck, just whatever," she added.

Jones later continued, "We go all out for each other here and we really, really love coming to work and being with each other."

Molly C. Quinn, who started out on the show when she was just 14 years old, talked to OTRC about how Nathan Fillion has become a sort of father-figure in her life.

"This is my family. This is my home. It's been incredible. Everything, high school, driving lessons, first boyfriend. I remember Nathan saying, 'I have to meet him,' and I did! I brought him to set and Nathan met my first boyfriend and my second and my third and my fourth and my fifth ... every one," Quinn said with a laugh. "They cycle quite a bit but he's always been there for me, you know, he's an awesome sounding board."